1. Field of the Invention
This invention relates to an oven and method for heating thermoplastic articles; more particularly, it is an oven and method for heating thermoplastic sheet for further shaping.
2. Description of the Prior Art
It is known that many thermoplastic polymers can be formed into shaped articles by various sheet metal forming techniques. These forming processes are economically very attractive because it is possible to achieve rapid production rates, e.g., rates that exceed one item a minute.
In one process, a thermoplastic sheet, reinforced with a fiber glass mat, woven cloth, or woven roving is preheated in an oven to above the softening point of the resin. The heated sheet, or blank, is transferred to a stamping press or compression molding press containing a matched metal die and formed into a shaped article. The article is then ejected from the die and removed from the press.
Difficulties have been encountered in ovens for heating thermoplastic articles. These difficulties relate to the uniform heating of the thermoplastic article and the handling of the thermoplastic articles as they are heated. Of particular concern are thermoplastic sheets used as stampable blanks. Some sheets become limp and difficult to handle when they are heated to temperatures above their softening point. Additionally, the sheets tend to stick to the supporting means in the oven. In order to compensate for the difficulties in handling of limp sheets and sheets that stick, ovens have been designed which compromise the uniform heating of the sheet.
One attempt to compensate for the sticking of the limp thermoplastic sheets to a conveyor belt which conveys said sheets through an oven has been disclosed in U.S. Pat. No. 4,181,551. In this patent, sheets are conveyed through an oven partially or totally by a fiberglass veil which eventually becomes incorporated into the sheet during the molding or stamping process. The thermoplastic sheet has a configuration in which a fiberglass veil is located at one surface of the molded product. The fiberglass veil affects the uniform heating, especially when infrared heating is used.
Other attempts have been made to avoid the sticking and handling problems and still maintain a uniform heating of the thermoplastic sheets or blanks. One such attempt is to place the blanks on metal plates, preferably aluminum coated with tetrafluoroethylene resin (TFE), and use a conveyor belt to convey the blanks on the plates from the inlet to the outlet of the oven. In this embodiment the metal plate carries the thermoplastic sheet or blank, and is itself conveyed on the conveyor belt through the oven. The metal plate is heated and, depending on its heat transfer characteristics, uniformly heats the supported side of the thermoplastic sheet.
Typically, thermoplastic sheets are conveyed on conventional metal link conveyor belts. These conveyor belts are comprised of metal strips, cables, bands, links, etc., connected together. There are openings between connected and adjacent metal pieces. The thermoplastic sheet or blank is supported on the linked metal conveyor. A heat source beneath the conveyor belt, such as an infrared heater, heats the supported surface, i.e., the surface in contact with the supporting belt of the thermoplastic article. As the thermoplastic article softens, it takes the impression of the conventional linked conveyor belt surface upon which it is supported. Further, the metal portions of the conveyor belt result in nonuniform heat transfer to the sheet.
It is desirable to provide an oven in which thermoplastic articles such as sheets or blanks can be heated uniformly with a minimum of sticking and handling problems resulting from the softness and limpness of the thermoplastic article as it is heated.